This invention relates to a synchronous motor control device. Since a synchronous motor is capable of having its power factor optionally controlled by controlling the field current, it has gained a wide application in driving equipment having relatively constant load or speed such as a kiln, a blower and the like. One of the primary reasons for driving the load of this kind by the synchronous motor is to control the power factor to an optional value and to improve the overall power factor including other loads. Accordingly, it is essential that a control device of a synchronous motor be capable of quickly detecting the change in the power factor of the synchronous motor so that it can supply a suitable field current control signal.
This power factor detection has conventionally been carried out by comparing the phase relationship between a predetermined voltage and current of the synchronous motor and obtaining a pulse signal that continues during the period of the predetermined phase relationship, e.g., the period in which both have the positive polarity, to make use of them as a power factor signal. As typical prior art of this type, mention can be made of Japanese Patent Publication No. 2488/1968. Accordingly, a signal representing the power factor can be obtained as a pulse signal only once a cycle (or a half cycle) of the electric power source of the synchronous motor.
The control device takes an average of this pulse signal by use of a smoothing circuit and uses the mean value as the power factor signal. For this reason, the conventional control device has large response delay due to the smoothing circuit and fails to sufficiently exhibit satisfactory control functions.